Vertex Shader Tricks by Bill Bilodeau - AMD at GDC14

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Bill explains some of the ways that the Vertex Shader can be used to improve performance by taking a fast path through the Vertex Shader rather than generating vertices with other parts of the pipeline in this AMD technology presentation from the 2014 Game Developers Conference in San Francisco March 17-21. Check out more technical presentations at http://developer.amd.com/resources/documentation-articles/conference-presentations/

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  • The value of SV_VertexID depends on the draw call. For non-indexed Draw, the vertex ID starts with 0 and increments by 1 for every vertex processed by the shader. For DrawIndexed(), the vertexID is the value of the index in the index buffer for that vertex.
  • For indexed Draw calls, create an index buffer which contains (index location + index number). That way you can calculate (vertexID/vertsPerMesh) to get the instance index, and (vertexID % vertsPerMesh) to get the index value which you can use to look up the vertex.
  • - If the mesh is being reused many times, then calculating the bounding box has little overhead.Bounding box can be used for collision detection
  • Could read and write from the UAV instead of binding an input SRV
  • Vertex Shader Tricks by Bill Bilodeau - AMD at GDC14

    1. 1. Vertex Shader Tricks New Ways to Use the Vertex Shader to Improve Performance Bill Bilodeau Developer Technology Engineer, AMD
    2. 2. Topics Covered ● Overview of the DX11 front-end pipeline ● Common bottlenecks ● Advanced Vertex Shader Features ● Vertex Shader Techniques ● Samples and Results
    3. 3. Graphics Hardware DX11 Front-End Pipeline ● VS –vertex data ● HS – control points ● Tessellator ● DS – generated vertices ● GS – primitives ● Write to UAV at all stages ● Starting with DX11.1 Vector GPR’s (256 2048-bit registers) Vector ALU (1 64-way single precision operation every 4 clocks) Scalar ALU (1 operation every 4 clocks) Scalar GPR’s (256 64-bit registers) Vector/Scalar cross communication bus Vector GPR’s (256 2048-bit registers) Vector ALU (1 64-way single precision operation every 4 clocks) Scalar ALU (1 operation every 4 clocks) Scalar GPR’s (256 64-bit registers) Vector/Scalar cross communication bus Vector GPR’s (256 2048-bit registers) Vector ALU (1 64-way single precision operation every 4 clocks) Scalar ALU (1 operation every 4 clocks) Scalar GPR’s (256 64-bit registers) Vector/Scalar cross communication bus . . . Input Assembler Hull Shader Domain Shader Tessellator Geometry Shader Stream Out CB, SRV, or UAV Vertex Shader
    4. 4. Bottlenecks - VS ● VS Attributes ● Limit outputs to 4 attributes (AMD) ●This applies to all shader stages (except PS) ● VS Texture Fetches ● Too many texture fetches can add latency ●Especially dependent texture fetches ●Group fetches together for better performance ●Hide latency with ALU instructions
    5. 5. Bottlenecks - VS ● Use the caches wisely ● Avoid large vertex formats that waste pre-VS cache space ● DrawIndexed() allows for reuse of processed vertices saved in the post-VS cache ●Vertices with the same index only need to get processed once Vertex Shader Pre-VS Cache (Hides Latency) Input Assembler Post-VS Cache (Vertex Reuse)
    6. 6. Bottlenecks - GS ● GS ● Can add or remove primitives ● Adding new primitives requires storing new vertices ●Going off chip to store data can be a bandwidth issue ● Using the GS means another shader stage ●This means more competition for shader resources ●Better if you can do everything in the VS
    7. 7. Advanced Vertex Shader Features ● SV_VertexID, SV_InstanceID ● UAV output (DX11.1) ● NULL vertex buffer ● VS can create its own vertex data
    8. 8. SV_VertexID ● Can use the vertex id to decide what vertex data to fetch ● Fetch from SRV, or procedurally create a vertex VSOut VertexShader(SV_VertexID id) { float3 vertex = g_VertexBuffer[id]; … }
    9. 9. UAV buffers ● Write to UAVs from a Vertex Shader ● New feature in DX11.1 (UAV at any stage) ● Can be used instead of stream-out for writing vertex data ● Triangle output not limited to strips ●You can use whatever format you want ● Can output anything useful to a UAV
    10. 10. NULL Vertex Buffer ● DX11/DX10 allows this ● Just set the number of vertices in Draw() ● VS will execute without a vertex buffer bound ● Can be used for instancing ● Call Draw() with the total number of vertices ● Bind mesh and instance data as SRVs
    11. 11. Vertex Shader Techniques ● Full Screen Triangle ● Vertex Shader Instancing ● Merged Instancing ● Vertex Shader UAVs
    12. 12. Full Screen Triangle ● For post-processing effects ● Triangle has better performance than quad ● Fast and easy with VS generated coordinates ● No IB or VB is necessary ● Something you should be using for full screen effects Clip Space Coordinates (-1, -1, 0) (-1, 3, 0) (3, -1, 0)
    13. 13. Full Screen Triangle: C++ code // Null VB, IB pd3dImmediateContext->IASetVertexBuffers( 0, 0, NULL, NULL, NULL ); pd3dImmediateContext->IASetIndexBuffer( NULL, (DXGI_FORMAT)0, 0 ); pd3dImmediateContext->IASetInputLayout( NULL ); // Set Shaders pd3dImmediateContext->VSSetShader( g_pFullScreenVS, NULL, 0 ); pd3dImmediateContext->PSSetShader( … ); pd3dImmediateContext->PSSetShaderResources( … ); pd3dImmediateContext->IASetPrimitiveTopology( D3D11_PRIMITIVE_TOPOLOGY_TRIANGLELIST ); // Render 3 vertices for the triangle pd3dImmediateContext->Draw(3, 0);
    14. 14. Full Screen Triangle: HLSL Code VSOutput VSFullScreenTest(uint id:SV_VERTEXID) { VSOutput output; // generate clip space position output.pos.x = (float)(id / 2) * 4.0 - 1.0; output.pos.y = (float)(id % 2) * 4.0 - 1.0; output.pos.z = 0.0; output.pos.w = 1.0; // texture coordinates output.tex.x = (float)(id / 2) * 2.0; output.tex.y = 1.0 - (float)(id % 2) * 2.0; // color output.color = float4(1, 1, 1, 1); return output; } Clip Space Coordinates (-1, -1, 0) (-1, 3, 0) (3, -1, 0)
    15. 15. VS Instancing: Point Sprites ● Often done on GS, but can be faster on VS ● Create an SRV point buffer and bind to VS ● Call Draw or DrawIndexed to render the full triangle list. ● Read the location from the point buffer and expand to vertex location in quad ● Can be used for particles or Bokeh DOF sprites ● Don’t use DrawInstanced for a small mesh
    16. 16. Point Sprites: C++ Code pd3d->IASetIndexBuffer( g_pParticleIndexBuffer, DXGI_FORMAT_R32_UINT, 0 ); pd3d->IASetPrimitiveTopology( D3D11_PRIMITIVE_TOPOLOGY_TRIANGLELIST ); pd3dImmediateContext->DrawIndexed( g_particleCount * 6, 0, 0);
    17. 17. Point Sprites: HLSL Code VSInstancedParticleDrawOut VSIndexBuffer(uint id:SV_VERTEXID) { VSInstancedParticleDrawOut output; uint particleIndex = id / 4; uint vertexInQuad = id % 4; // calculate the position of the vertex float3 position; position.x = (vertexInQuad % 2) ? 1.0 : -1.0; position.y = (vertexInQuad & 2) ? -1.0 : 1.0; position.z = 0.0; position.xy *= PARTICLE_RADIUS; position = mul( position, (float3x3)g_mInvView ) + g_bufPosColor[particleIndex].pos.xyz; output.pos = mul( float4(position,1.0), g_mWorldViewProj ); output.color = g_bufPosColor[particleIndex].color; // texture coordinate output.tex.x = (vertexInQuad % 2) ? 1.0 : 0.0; output.tex.y = (vertexInQuad & 2) ? 1.0 : 0.0; return output; }
    18. 18. Point Sprite Performance Indexed, 500K SpritesNon-Indexed, 500K SpritesGS, 500K SpritesDrawInstanced, 500K SpritesIndexed, 1M SpritesNon-Indexed, 1M SpritesGS, 1M SpritesDrawInstanced, 1M Sprit R9 290x (ms) 0.52 0.77 1.38 1.77 1.02 1.53 2.7 3.54 Titan (ms) 0.52 0.87 0.83 5.1 1.5 1.92 1.6 10.3 0 2 4 6 8 10 12 AMD Radeon R9 290x Nvidia Titan
    19. 19. Point Sprite Performance ● DrawIndexed() is the fastest method ● Draw() is slower but doesn’t need an IB ● Don’t use DrawInstanced() for creating sprites on either AMD or NVidia hardware ● Not recommended for a small number of vertices
    20. 20. Merge Instancing ● Combine multiple meshes that can be instanced many times ● Better than normal instancing which renders only one mesh ● Instance nearby meshes for smaller bounding box ● Each mesh is a page in the vertex data ● Fixed vertex count for each mesh ●Meshes smaller than page size use degenerate triangles
    21. 21. Merge Instancing Mesh Vertex Data Mesh Data 0 Mesh Data 1 Mesh Data 2 . . . Mesh Instance Data Instance 0 Mesh Index 2 Instance 1 Mesh Index 0 . . . Degenerate Triangle Vertex 0 Vertex 1 Vertex 2 Vertex 3 . . . 0 0 0 Fixed Length Page
    22. 22. Merged Instancing using VS ● Use the vertex ID to look up the mesh to instance ● All meshes are the same size, so (id / SIZE) can be used as an offset to the mesh ● Faster than using DrawInstanced()
    23. 23. Merge Instancing Performance 0 5 10 15 20 25 30 DrawInstanced Soft Instancing R9 290x GTX 780 ● Instancing performance test by Cloud Imperium Games for Star Citizen ● Renders 13.5M triangles (~40M verts) ● DrawInstanced version calls DrawInstanced() and uses instance data in a vertex buffer ● Soft Instancing version uses vertex instancing with Draw() calls and fetches instance data from SRV AMD Radeon R9 290X Nvidia GTX 780 ms
    24. 24. Vertex Shader UAVs ● Random access Read/Write in a VS ● Can be used to store transformed vertex data for use in multi-pass algorithms ● Can be used for passing constant attributes between any shader stage (not just from VS)
    25. 25. Skinning to UAV ● Skin vertex data then output to UAV ● Instance the skinned UAV data multiple times ● Can also be used for non-instanced data ● Multiple passes can reuse the transformed vertex data – Shadow map rendering ● Performance is about the same as stream-out, but you can do more …
    26. 26. Bounding Box to UAV ● Can calculate and store Bbox in the VS ● Use a UAV to store the min/max values (6) ● InterlockedMin/InterlockedMax determine min and max of the bbox ●Need to use integer values with atomics ● Use the stored bbox in later passes ● GPU physics (collision) ● Tile based processing
    27. 27. Bounding Box: HLSL Code void UAVBBoxSkinVS(VSSkinnedIn input, uint id:SV_VERTEXID ) { // skin the vertex . . . // output the max and min for the bounding box int x = (int) (vSkinned.Pos.x * FLOAT_SCALE); // convert to integer int y = (int) (vSkinned.Pos.y * FLOAT_SCALE); int z = (int) (vSkinned.Pos.z * FLOAT_SCALE); InterlockedMin(g_BBoxUAV[0], x); InterlockedMin(g_BBoxUAV[1], y); InterlockedMin(g_BBoxUAV[2], z); InterlockedMax(g_BBoxUAV[3], x); InterlockedMax(g_BBoxUAV[4], y); InterlockedMax(g_BBoxUAV[5], z); . . .
    28. 28. Particle System UAV ● Single pass GPU-only particle system ● In the VS: ● Generate sprites for rendering ● Do Euler integration and update the particle system state to a UAV
    29. 29. Particle System: HLSL Code uint particleIndex = id / 4; uint vertexInQuad = id % 4; // calculate the new position of the vertex float3 oldPosition = g_bufPosColor[particleIndex].pos.xyz; float3 oldVelocity = g_bufPosColor[particleIndex].velocity.xyz; // Euler integration to find new position and velocity float3 acceleration = normalize(oldVelocity) * ACCELLERATION; float3 newVelocity = acceleration * g_deltaT + oldVelocity; float3 newPosition = newVelocity * g_deltaT + oldPosition; g_particleUAV[particleIndex].pos = float4(newPosition, 1.0); g_particleUAV[particleIndex].velocity = float4(newVelocity, 0.0); // Generate sprite vertices . . .
    30. 30. Conclusion ● Vertex shader “tricks” can be more efficient than more commonly used methods ● Use SV_Vertex ID for smarter instancing ●Sprites ●Merge Instancing ● UAVs add lots of freedom to vertex shaders ●Bounding box calculation ●Single pass VS particle system
    31. 31. Demos ● Particle System ● UAV Skinning ● Bbox
    32. 32. Acknowledgements ● Merge Instancing ● Emil Person, “Graphics Gems for Games” SIGGRAPH 2011 ● Brendan Jackson, Cloud Imperium ● Thanks to ● Nick Thibieroz, AMD ● Raul Aguaviva (particle system UAV), AMD ● Alex Kharlamov, AMD
    33. 33. Questions ● bill.bilodeau@amd.com
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